OPF-007 is reported as warning by default. The EPUB is still valid, but something is likely to cause problems in some reading apps. EPUBCheck exits with 0 unless you pass --failonwarnings.

EPUBCheck · Package document (OPF)

`OPF-007`

Updated: Nov 3, 2026

EPUBCheck OPF-007 is a warning message: “Re-declaration of reserved prefix "…".” The package document re-declares one of EPUB's reserved prefixes.

The official message

Re-declaration of reserved prefix "%1$s".

Default severity: Warning. The EPUB is still valid, but something is likely to cause problems in some reading apps. EPUBCheck exits with 0 unless you pass --failonwarnings.
EPUBCheck version: 5.4.0

%1$s, %2$s… are filled in with the file, value or element EPUBCheck found.

Typical causes

Copying a prefix attribute that includes mappings for reserved prefixes such as dcterms or rendition.
Hand-edited or tool-generated package documents that write the prefix attribute with the wrong separators.

How to fix it

The prefix attribute on the package element maps short prefixes to vocabulary URIs, as space-separated pairs like prefix="ibooks: http://vocabulary.itunes.apple.com/rdf/ibooks/vocabulary-extensions-1.0/". Remove the mapping for the reserved prefix from the attribute.
Reserved prefixes such as dcterms, rendition and schema need no declaration at all.

Terms used here

Other OPF messages

OPF-006 Invalid prefix declaration: URI "…" is not a valid URI.
OPF-005 Invalid prefix declaration: URI for prefix "…" doesn’t exist.
OPF-004 Invalid prefix declaration: leading or trailing whitespace is not allowed.
OPF-004a Invalid prefix declaration: found empty prefix.
OPF-004b Invalid prefix "…": must be a valid non-colonized name (aka "NCName").
OPF-004c Invalid prefix declaration: prefix "…" must be immediately followed by a colon character (":").

Message text and severity from EPUBCheck 5.4.0 (W3C, BSD-3-Clause). Explanation by EPUBLab.